Related art wireless systems may involve user equipment (UE) such as a mobile phone, laptop or PDA that interact with a centralized entity such as a Base Station (BS). BS's may be cellular base stations or wireless local area network (WLAN) Access points (AP). FIG. 1 illustrates an exemplary configuration of a base station with downlink and uplink transmissions. Transmission 101 from the BS 100 to the UE 103 is known as a downlink (DL) transmission, and transmission 104 from the UE 105 to the BS 100 is known as an uplink (UL) transmissions. From the point of view of the UE, data is received from the BS via a DL channel 102, and data is transmitted to the BS via a UL channel 106.
The link gain (loss) Loss of a wireless channel depends on several factors and can be expressed as:Loss=PL+S+F+GAnt+LMisc  (1)where PL is the path loss, S is the large scale shadow fading, F is the small scale multipath fading, GAnt, is the antenna gain factor and LMisc are other the miscellaneous gain factors. The path loss and fading parameters are dependent on the frequency of the signal being transmitted.
The first action of a UE when the UE is switched on is to associate with a BS. The UE measures signals generated by the BS. These signals may be synchronization symbols for cellular systems and AP beacons in WLAN systems, and are known as Reference Signals (RS). Each BS transmits RS's in the DL channel periodically. The received signal strength at the UE is called the RS received power (RSRP) and is expressed in terms of the transmit power P, of the BS, as:RSRPi=Lossi+Pi  (2)The UE measures RSRPi of each BSi and may associate with the BS with the maximum value. UE association is based on the DL channel RSRP, which depends on the DL channel link gain. However, the UE subsequently sends to the same BS for both UL and DL transmissions. If the UL channel from the UE to the BS is the strongest channel amongst the set of available UL channels to different BSs, then the same BS may be utilized for both UL and DL transmissions.
However, DL-UL imbalance in link gains is not an uncommon problem. In Third Generation Partnership Project (3GPP) Universal Mobile Telecommunication Systems (UMTS), a UE in a soft handoff situation may handle a DL-UL imbalance to a certain extent. The serving cell may have the stronger DL, but the UL to the non-serving cell may be stronger than the UL to the serving cell. Both 3GPP Release 99 (R99) and High Speed Uplink Packet Access (HSUPA) sessions may exploit the inherent diversity from soft handoff to operate in the presence of some DL-UL imbalance. However, with High Speed Downlink Packet Access (HSDPA) and Long Term Evolution (LTE), the UL to the serving cell affects feedback control information. HSDPA and LTE throughput may be severely impacted as a result of a DL-UL imbalance. One way to mitigate the impact is to change serving cells based not just on DL quality but UL quality as well. However, the problem is more serious in heterogeneous 3GPP networks settings, where different BS;s can have different transmit powers.